Grinding elements are freely movable bodies with whose help a raw material is pulverized in mills. They have been known for a long time and can consist of wear-resistant steel, porcelain, flint or ceramic oxide. Grinding elements made of ceramic oxide materials are free of clay as a rule, because they can only meet the high requirements set on grinding elements in this condition. It is known that organic binders are used in place of clay.
In accordance with the state-of-the-art of technology, grinding elements are made of ceramic oxide, i.e., such as in a spherical form, from finely ground fused corundum or calcined alumina. The corundum powder (aluminum oxide powder) mixed with a binder is brought into a spherical form through isostatic or axial cold pressing. The spheres, possibly after drying, are then converted into grinding elements by firing, whereby the relatively soft blanks are converted into fixed and hard, particularly wear-resistant grinding elements.
Today, the firing of the blanks most often takes place in a tunnel kiln with gas, oil or electric heating, in which they are pushed along on carriages through a duct with stationary preheating, firing and cooling zones, whereby use is made of firing accessories, particularly firing capsules. The grinding elements are then removed from the firing capsules and sent to their application. The firing capsules, which can also consist of corundum, are filled again with blanks and pushed through the oven again on carriages. According to experience, the firing capsules must be replaced after approximately 40 to 50 passes through the kiln.
A particular disadvantage of the known process is considered to be the fact that firing in tunnel kilns is very work and cost intensive. The replacement of the firing capsules after 40 to 50 passes through the kiln, like their filling before the firing process and their emptying after the firing process, is considered to be technically out-of-date and no longer contemporary, especially with respect to the costs and energy required.
A disadvantage of the grinding elements manufactured in accordance with the current state-of-the-art of technology is often considered to be the fact that they contain only approximately 92 Wt. % aluminum oxide and are thereby "heavily" contaminated. The hardness and wear-resistance of the grinding elements therefor do not satisfy the highest requirements.
It thus became a problem to find a process for the manufacturing of grinding elements through the forming of ground aluminum oxide and subsequent firing of the molded pieces, in which the use of firing accessories was not necessary during the firing process. The formed grinding elements should be fed into the kiln loosely after their manufacturing.
A further purpose of the present invention consists in finding a manufacturing process whose costs lie significantly below those of the manufacturing process corresponding to the state-of-the-art of technology. Furthermore, the new process should be usable with aluminum oxide of higher purities, for example with aluminum oxide having an aluminum oxide content of 99 Wt. % and more. With this type of aluminum oxide, which can be manufactured on an industrial scale by the calcination of aluminum hydroxide and/or recrystallization of aluminum oxide, contamination of the raw material in the grinder through the wear of the grinding elements can be avoided. Aluminum oxide of this type is also designated as sintered alumina.